A liquid crystal display device is increasingly used year by year for a space-saving and less power-consuming image display device. It has been conventionally a drawback of the liquid crystal display device that the viewing-angle dependency of the displayed image is large, but a wide viewing-angle liquid crystal mode so-called VA mode is put into practical use and this allows rapid expansion of the demand for a liquid crystal display device also in the market where a high-quality image is required, such as television. A VA-mode liquid crystal display device is advantageously assured of high contrast as compared with other liquid crystal display modes but has a problem that the contrast and color tint are greatly changed depending on the viewing angle.
As a solution for this problem, a method of using a phase difference film having reverse dispersibility (also referred to as positive wavelength dispersion characteristics) in which a phase difference is larger as a wavelength increases is effectively used. For example, WO 08/102,647 pamphlet discloses a method of using two biaxially-oriented films in which both in-plane retardation (Re) and retardation for the thickness direction (Rth) show reverse dispersibility, and JP-A-2006-291192 (“JP-A” means unexamined published Japanese patent application) discloses a method of using two biaxially-oriented films, in which Re shows reverse dispersibility and Rth shows forward dispersibility, in combination.
Cellulose acylate films, if they are composed of a single polymer as film-constituting polymer, both Re and Rth have reverse dispersibility and are thus favorable for the object above. Because the cellulose acylate films are highly transparent and readily adhesive to polyvinylalcohol that is used as polarizer, they have been widely used as a polarizing plate-protective film. Recently, under demand for reduction in a thickness of liquid crystal display devices, studies aimed at providing such a cellulose acylate film with functions of both a polarizing plate-protective film and a phase difference film by providing it with phase difference characteristics, and additionally making the film itself thinner are under progress. However when a film made only of cellulose acylate is made thinner, the film shows increased moisture permeability, and thus, causing a problem that the display performance of the resulting liquid crystal display device deteriorates under high temperature and high humidity.
A method of adding a compound more hydrophobic than cellulose acylate to the film is studied to overcome the problem, and, specifically, JP-A-2003-183417 discloses a method of adding a ketone resin. However, these compounds also have a problem that the decrease in moisture permeability of the cellulose acylate film is not sufficient or the reverse dispersibility of retardation are reduced, and there exist needs for improvement.